There are semiconductor lasers having a current constricting structure in order to improve the current flow efficiency. A vertical-cavity surface-emitting laser (VCSEL) is an example of such a semiconductor laser. The VCSEL emits light in a direction perpendicular to a substrate, and compared to the so-called edge-emitting semiconductor lasers, the VCSEL has low cost, low power consumption, small size, high performance and is suited for application to two-dimensional devices. For this reason, much attention is recently drawn to the VCSEL.
An AlAs selective oxidation constricting structure is popularly used for the current constricting structure of the VCSEL, as may be seen from a U.S. Pat. No. 5,493,577, for example. This current constricting structure of the VCSEL is formed by placing a semiconductor substrate or a semiconductor sample including a circular or rectangular base shaped mesa structure within a high-temperature steam atmosphere, and oxidizing a p-AlAs selective oxidized layer included in the mesa structure from an outer peripheral edge that is exposed at a side surface of the mesa structure towards a central portion in a state leaving the central portion non-oxidized so as to form an AlxOy current constricting part (oxidized region). In the VCSEL having the AlxOy current constricting part that is formed in the above described manner, the index of refraction of the AlxOy current constricting part is approximately 1.6, and is low compared to indexes of refraction of other semiconductor layers. For this reason, a difference is introduced between the indexes of refraction in a lateral direction within a resonant structure, and it is possible to trap the light at the center of the mesa structure. Thus, desirable characteristics are obtained in that the current constricting efficiency of the semiconductor element is good and the threshold current is low.
In order to obtain the single basic lateral mode oscillation in the VCSEL, it is necessary to make the size (for example, the diameter) of the current constricting part must be made small and the diffraction loss with respect to the high order mode must be made large. More particularly, the size of one side or the diameter of the current constricting part must be made narrow to approximately 3 to approximately 4 times the oscillation wavelength. For example, if the oscillation wavelength is 0.85 μm or 1.3 μm, the size of one side or the diameter of the current constricting part must be approximately 3.5 μm or less or approximately 5.0 μm or less, respectively.
A semiconductor oxidation apparatus satisfying such needs is proposed in Zenno et al., “Development of New Oxidation Apparatus For Manufacturing Surface-Emitting Lasers”, Optical Alliance, pp. 42-46, April 2004. FIG. 1 shows a general structure of this proposed semiconductor oxidation apparatus. A semiconductor oxidation apparatus 1010 shown in FIG. 1 has a sealable container (or oxidation chamber) 1012, and a heating stage 1016 that has a built-in heater is provided in a chamber interior 1014 of this oxidation chamber 1012. A substrate holder 1018 is provided on the heating stage 1016, and a semiconductor sample (or semiconductor substrate) 1020 is placed on the substrate holder 1018. The oxidation chamber 1012 is also provided with an inlet pipe 1022 for supplying an oxidizing atmosphere including vapor into the chamber interior 1014, and an exhaust pipe 1024 for exhausting the oxidizing atmosphere within the chamber interior 1014 after an oxidation process ends. According to the semiconductor oxidation apparatus 1010 having such a structure, it is possible to uniformly oxidize the semiconductor sample 1020 with a relatively good reproducibility. However, the amount of oxidation of the semiconductor sample 1020 is affected by inconsistencies among the lots, such as the composition and the film thickness after the crystal growth in a semiconductor film forming apparatus. Particularly in the case of a semiconductor layer including Al and As, the film thickness and the AlAs composition are extremely sensitive to the oxidation temperature and the like as described in Choquette et al., “Advances in Selective Wet Oxidation of AlGaAs Alloys”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, No. 3, pp. 916-926, June 1997, and are also affected by the thickness of a natural oxidation layer that is formed on a side surface of a non-oxidized layer of the semiconductor sample immediately prior to the oxidation process. As a result, the size of the current constricting part causes the inconsistency in the oscillation characteristic such as the optical output, and the yield deteriorates. Particularly in the case of a single-mode element in which the absolute value of the area of the current constricting part is small compared to that of a multi-mode element, the effects of the inconsistency in the amount of oxidation on the inconsistency of the element characteristics are extremely large, and if the area of the current constricting part becomes large the element that should originally behave as a single-mode element may behave like a multi-mode element.
In order to solve the problems described above, methods of monitoring the amount of oxidation during the oxidation process have been proposed in Feld et al., “In Situ Optical Monitoring of AlAs Wet Oxidation Using a Novel Low-Temperature Low-Pressure Steam Furnace Design”, IEEE Photonics Technology Letters, Vol. 10, No. 2, pp. 197-199, February 1998 and a Japanese Laid-Open Patent Application No. 2003-179309. According to the proposed methods, a semiconductor sample 1020 during the oxidation process is monitored by a microscope 1028 via a monitoring window 1026 as shown in FIG. 2. The oxidation distance or the area of the non-oxidized region (oxidation rate) is estimated from the contrast between the oxidized region and the non-oxidized region that are monitored by the microscope 1028, and the amount of oxidation is thereafter controlled based on the estimated oxidation distance or the area of the oxidation rate. But normally, the mesa diameter of the VCSEL is approximately 10 μm to approximately 50 μm, and the magnification of the microscope 1028 must be set to approximately 1000 times in order to strictly control the diameter of the current constricting part. In addition, in order to match the focal point of the microscope 1028 on the mesa, a distance L1 between the semiconductor sample 1020 and the monitoring window 1026 and a distance L2 between the monitoring window 1026 and the microscope 1028 must be set short as possible. However, if the distance L1 between the semiconductor sample 1020 and the monitoring window 1026 during the oxidation process is set short, local inconsistencies are generated in the vapor density distribution on the semiconductor sample 1020 and the temperature distribution on the semiconductor sample 1020, to thereby generate an in-plane distribution of the amount of oxidation and cause a deterioration in the yield. On the other hand, if the distance L2 between the monitoring window 1026 and the microscope 1028 is set short, the index of refraction of the monitoring window 1026 may change due to the heat generated from the heater, and the optical elements such as a lens assembled in the microscope 1028 may undergo a thermal deformation and generate a shift in the focal point, to thereby deteriorate the measuring accuracy.